I have one of these mills- http://www.lathemaster.com/SIEG%20X3%20MILLING%20MACHINE.htm can I get a new milling head with the upgraded head for it? Is it a direct replacment?

You can.... however..... Not advised. Sieg mates each head, table, saddle, base with each other. I would expect that some "fit and finish" will be required for acceptable working condition. With the cost of the machines, it would be more prudent to sale the machine and purchase another.

Just my oppinion though.